The solubility in water or organic solvents of salts of anionic dyes and fluorescent brighteners depends not only on the structure of the dye or brightener, but especially also on the particular salt form, i.e. on the cation. For instance, the sodium salts generally obtained on synthesis are in many cases not readily water-soluble, whereas as lithium salts the same dyes or brighteners are significantly more so (see, for example, German Pat. No. 1,644,308).
To prepare concentrated dye or brightener solutions, which are becoming commercially increasingly important as liquid brands, it is necessary to have dye or brightener salts which are readily water-soluble, since it is only in this way that the solvent content in the preparations can be kept as low as possible. But a high solubility of the dye or brightener is also of advantage in preparing solid preparations, since it makes possible a gentle and energy-saving drying, because highly concentrated starting solutions can be used.
In the past there has been no shortage of attempts to replace the cations of sparingly soluble dye or brightener salts and thus to increase the water-solubility of the salts. Stilbeneazo(xy) dyes, for example, can at present be treated by a whole number of different processes, for example by selective precipitation of sodium ions by means of hexafluorosilicic acid in the presence of alkanolamine (German Offenlegungsschrift 2,451,219) or double resalting involving phase separation (European Patent A 0,053,220). German Offenlegungsschrift 2,805,891 proposes that cation or anion exchange be carried out by means of a membrane-separating method, for example diafiltration. However, this method is only suitable for resaltings, for example of sulfur-containing dyes, if carried out in the presence of cations whose molecular weight is at least equal to or greater than that of the dyes, because otherwise the dyes would also be separated out of the dye solution.
The object is to find a process which is simple to carry out and which places no particular requirements on the molecular weight of the cations to be exchanged.
This object is surprisingly achieved by means of Donnan dialysis (on the theory of the Donnan dialysis, see S. T. Hwang and K. Kammermeyer, Membranes in Separations; Wiley & Sons, New York 1975).
The present invention thus relates to a process for converting sparingly soluble salts of dyes and fluorescent brighteners into more soluble salts by means of cation exchange without intermediate isolation of the free dye acid, the cation exchange being carried out by means of Donnan dialysis.